Protein Phosphorylation And regulation of Cytoskeleton in Nervous system The topographic regulation by phosphorylation of neuronal cytoskeleton proteins is tightly regulated in the nervous system. Cytoskeletal proteins are selectively phosphorylated in the axonal compartment in lys/ser/pro (KSP) motifs of their carboxy tail domains but not in the cell body. Deregulation of this post translational modification has been implicated in various neurodegenerative diseases. We have proposed the following hypotheses to explain the topographic regulation of cytoskeletal proteins in the nervous system under physiological conditions. After biosynthesis in the cell bodies, CaMK/ PKA/PKC transiently phosphorylate the cytoskeletal proteins in the N-terminal domains. By virtue of conformational change induced by this phosphorylation, the phosphorylation in the C-terminal domains by proline directed kinases (Cdk5, MAPKs) is inhibited. Exogenous signals, either from the target tissues or from surrounding axon-associated glia , activate the proline -directed kinases which extensively phosphorylate the proline-directed S/T residues in the axonal compartment; and the differential expression of kinases and phosphatase activities in the cell body and axonal compartments. Our in situ and in vitro experiments have provided the evidence that as long as head domains are phosphorylated, the phosphorylation of their tail domains on KSP motifs is inhibited. In vivo and in vitro studies have shown that glial / axonal interaction is responsible for MAPK activation and tail domain KSP phosphorylation; and to evaluate the kinase and phosphatase activities, we have used the squid giant fiber system where the axonal compartment can be separated from the cell body. We have found that the kinase and ser/thr phosphatase activities are similar in both compartments however, protein tyrosine phosphatase activity is significantly higher in the cell body compared to the axonal compartment. This may be one of the factors responsible for low phosphorylation of the cytoskeletal proteins in the cell body compartment in anormal system. The extracellular aggregation of amyloid b (Ab) peptides and the intracellular hyperphosphorylation of tau, a neuronal cytoskeletal protein at specific epitopes, are pathological hallmarks of neurodegenerative diseases such as Alzheimer's disease (AD). Cdk5 phosphorylates tau at AD-specific phospho-epitopes when it associates with p25. p25 is a truncated activator which is produced by the proteolytic cleavage of the physiological Cdk5 activator, p35, upon exposure to Ab and other neurotoxic insults. We showed that lentiviral infections of cortical neurons with a Cdk5 Inhibitory Peptide, CIP (a 125 amino acid peptide truncated form of p35) selectively inhibited p25/Cdk5 activity and suppressed the aberrant tau hyperphosphorylation in cortical neurons. Furthermore, Ab1-42-induced apoptosis of cortical neurons was also reduced by co-infection with CIP. Of particular importance, is our finding that CIP did not inhibit endogenous or transfected p35/Cdk5 activity, which is required for normal nervous system function and survival. CIP also did not inhibit endogenous Cdc2 activity, nor did it inhibit the other mitotic cyclin dependent kinases such as Cdk2, Cdk4 and Cdk6. It is highly specific for the hyperactive p25/Cdk5 complex seen in abnormally stressed neurons. These results, therefore, provide a strategy to address, and possibly ameliorate, the pathology of neurodegenerative diseases that may be a consequence of aberrant p25 activation of Cdk5, without affecting 'normal' Cdk5 activity.